<?php
/**
 * Created by PhpStorm.
 * User: CC
 * Date: 2016/3/9
 * Time: 23:39
 */

function buildRandomString($type=1,$length=4){
    if($type==1){
        $chars=join("",range(0,9));
    }elseif($type==2){
        $chars=join("",array_merge(range("a","z"),range("A","Z")));
    }elseif($type==3){
        $chars=join("",array_merge(range("a","z"),range("A","Z"),range(0,9)));
}

    if($length>strlen($chars)){
        exit("字符串长度不够");
    }
    $chars=str_shuffle($chars);
    return substr($chars,0,$length);
}